1. Field of the Invention:
The present invention relates to a feeding apparatus employing a ball screw in which a ball nut is fitted on a ball screw shaft through steel balls, and the ball nut and the ball screw shaft are rotated or axially moved relative to each other, whereby a movable table, such as a work table, which is connected to the ball nut is selectively moved in either a fine feed or quick traverse mode.
2. Description of the Prior Art:
A known feeding apparatus has heretofore been arranged as follows. A ball screw shaft is supported on a fixed bed in such a manner as to be movable in the direction of rotation thereof and immovable in the axial direction thereof. A stepping motor is connected to the ball screw shaft, and a ball nut is fitted on the ball screw shaft through steel balls. Further, a movable table is connected to the ball nut. With this arrangement, the ball screw shaft is rotated by the drive of the stepping motor such as to move the ball nut in the axial direction of the ball screw shaft, whereby the movable table connected to the ball nut is moved relative to the fixed bed.
Such a conventional feeding apparatus, however, suffers the following disadvantages.
As the ball screw shaft is rotated a predetermined angle as the result of the rotation of the stepping motor by one step (e.g., a rotational angle of 360/800=0.45.degree. in the case of a stepping motor with 800 steps), the ball nut is moved relative to the ball screw shaft in the axial direction thereof by a predetermined distance in correspondence to the lead of the threads which are respectively formed on the ball screw shaft and the ball nut. In this case, it may be possible to change over the feed mode of the movable table between a fine feed mode and a quick traverse mode by varying the lead angle of the threads, or by interposing a speed change gear between the stepping motor and the ball screw shaft and varying the amount of axial movement of the ball nut corresponding to a predetermined rotational angle of the motor by means of the speed change gear. However, if the lead of the threads is decreased for the purpose of obtaining a fine feed operation, then it becomes impossible to effect a quick traverse operation and it is also difficult to carry out a precise thread cutting operation; if the lead of the threads is increased, it becomes difficult to effect a fine feed operation. On the other hand, when a speed change gear is employed, the size of the apparatus as a whole becomes unfavorably large. In addition, it is extremely difficult to effect an accurate fine feed operation owing to unavoidable errors in production of the speed change gear itself. Further, when a stepping motor with a relatively large number of steps (i.e., a relatively small angle of rotation per step)is used to obtain a fine feed operation, the production cost of the apparatus as a whole is increased by a large margin since such a stepping motor with a large number of steps is extremely costly.
Furthermore, since the ball screw shaft and the motor are connected through a gear with a pair of gear wheels which are meshed with each other, any backlash between these gear wheels may make it difficult to accurately transmit the rotational driving force derived from the motor to the ball screw shaft. In addition, since the ball screw shaft per se has relatively large inertia, it is not possible to accurately position the movable table connected to the ball nut which is fitted on the ball screw shaft. Moreover, the above-described facts disadvantageously involve inferior responsiveness, that is, it is not possible for the ball screw shaft to quickly respond to a change in the rotational direction of the motor. Moreover, in the case where the ball screw shaft is rotated at high speed for the purpose of increasing the feed rate of the movable table, when the rotational speed of the ball screw shaft being thus increased exceeds a predetermined value (what is called critical speed), the ball screw shaft undesirably vibrates sympathetically. It is therefore necessary for the ball screw shaft to be rotated below an allowable rotational speed which is determined by the critical speed, which fact disadvantageously limits the range of quick traverse of the movable table.